Niobium suboxide powder

ABSTRACT

A niobium suboxide powder comprising 100 to 600 ppm of magnesium is described. The niobium suboxide powder may (alternatively or in addition to 100 to 600 ppm of magnesium) further include 50 to 400 ppm of molybdenum and/or tungsten. The niobium suboxide powder is suitable for the production of: capacitors having an insulator layer of niobium pentoxide; capacitor anodes produced from the niobium suboxide powder; and corresponding capacitors.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/899,719, filed Jul. 13, 2004, which claims the right of priorityunder 35 U.S.C. §119(a)-(d) of German Patent Application No. DE 103 31891.7, filed Jul. 15, 2003.

FIELD OF THE INVENTION

The invention relates to a niobium suboxide powder which is suitable forthe production of capacitors with a niobium pentoxide insulator layer,to capacitor anodes produced from the niobium suboxide powder, and tocorresponding capacitors.

Solid electrolyte capacitors with a very large active capacitor surfacearea and therefore a small overall construction suitable for mobilecommunications electronics used are predominantly capacitors with aniobium or tantalum pentoxide barrier layer applied to a correspondingconductive substrate, utilizing the stability of these compounds (“valvemetals”), the relatively high dielectric constant and the fact that theinsulating pentoxide layer can be produced with a very uniform layerthickness by electrochemical means. The substrates used are metallic orconductive lower oxide (suboxide) precursors of the correspondingpentoxides. The substrate, which simultaneously forms a capacitorelectrode (anode) comprises a highly porous, sponge-like structure whichis produced by sintering extremely fine-particle primary structures orsecondary structures which are already in sponge-like form. The surfaceof the substrate structure is electrolytically oxidized (“formed”) toproduce the pentoxide, with the thickness of the pentoxide layer beingdetermined by the maximum voltage of the electrolytic oxidation(“forming voltage”). The counterelectrode is produced by impregnatingthe sponge-like structure with manganese nitrate, which is thermallyconverted into manganese dioxide, or with a liquid precursor of apolymer electrolyte followed by polymerization. The electrical contactsto the electrodes are produced on one side by a tantalum or niobium wirewhich is sintered in during production of the substrate structure and onthe other side by the metallic capacitor sheath, which is insulated withrespect to the wire.

The capacitance C of a capacitor is calculated using the followingformula:

C=(F·∈)/(d·V _(F))

where F denotes the capacitor surface area, ∈ the dielectric constant, dthe thickness of the insulator layer per V of forming voltage and V_(F)the forming voltage. Since the dielectric constant ∈ is 27.6 or 41 fortantalum pentoxide or niobium pentoxide, respectively, but the growth inthe layer thickness per volt of forming voltage d is 16.6 or 25 Å/V,both pentoxides have the same quotient ∈/d=1.64 or 1.69, respectively.Capacitors based on both pentoxides, with the same geometry of the anodestructures, therefore have the same capacitance. Trivial differences indetails concerning specific weight-related capacitances result from thedifferent densities of Nb, NbO_(x) and Ta. Anode structures made from Nband NbO_(x) therefore have the advantage of saving weight when used, forexample, in mobile telephones, in which every gram of weight saving is apriority. With regard to cost aspects, NbO_(x) is more favourable thanNb, since some of the volume of the anode structure is provided byoxygen.

EP 1 388 870 A1 has already disclosed capacitors which include anelectrode produced by sintering a niobium suboxide powder of formulaNbO_(x) (x=0.8 to 1.2). The niobium suboxide powder described in EP 1388 870 A1 is distinguished in particular by a tap density of 0.5 to 2.5g/ml, and the sintered body produced therefrom is distinguished by aspecific porosity. The niobium suboxide powder may contain a largenumber of other elements, e.g. Mg, Ca, Sr, Ba, Sc, Y, La, Ce, Pr, Ti,Zr, Hf, V, Ta, Mo, W, Mn, Re, Ru, Os, Rh, Ir, Pd, Al, C, Si and numerousothers; these elements may be added in a quantity of from 50 to 200,000ppm, i.e. up to 20% by weight. EP 1388 870 A1 does not disclose whetherthe presence of certain elements belonging to the abovementioned grouphas particular advantages and in what quantity specific elements shouldbe present.

One significant quality feature of capacitors is the leakage current,i.e. the residual current permeability of the pentoxide barrier layer,which should be as low as possible in order to avoid losses. Impuritiessuch as Fe, Cr, Li, alkali metals, halogens, carbon and others haveparticularly adverse effects on the residual current. These impurities,in capacitors based on niobium suboxide, may evidently still have anadverse effect with regard to the residual current even in extremely lowconcentrations. When niobium suboxide is being produced by means of thestandard metallurgical process, in which highly oxidized niobium (Nb₂O₅)is treated with metallic niobium at elevated temperature in anonoxidizing, preferably reducing atmosphere so as to balance out theoxygen concentration, the metallic impurities of smaller atomic radiusevidently accumulate at the particle surface layer during the diffusionof the oxygen into the originally metallic particles, since they arefaster at performing the required site exchange reaction than theniobium atoms. After forming of the anode structure, they are thenavailable as imperfections in particular in the barrier layer. Themigration of impurities to the surface moreover is not symmetrical, butrather is uneven, for example depending on whether the random adjacentparticle happens to be an oxygen-donating or an oxygen-receivingparticle in the oxygen exchange. This causes fluctuations in theimpurity concentrations, which are associated with increased peakresidual current values.

SUMMARY OF THE INVENTION

It is an object of the present invention to reduce the residual currentin capacitors based on niobium suboxide anodes.

A further object of the invention is to reduce fluctuations in residualcurrent caused by the production of capacitors within a batch.

A further object of the invention is to provide a niobium suboxidepowder which is advantageous in terms of primary and secondary structurefor the capacitor production.

It has been discovered that by doping niobium suboxide with magnesium,tungsten and/or molybdenum, it is possible to have a favourable effecton the residual current of capacitors produced therefrom with regard tothe abovementioned properties. The doping elements incorporated in thelattice evidently form traps for disruptive impurities, i.e. in thevicinity of the doping elements, which form imperfections in the latticestructure of the niobium oxide, the impurities may be bonded in such amanner that they are neutralized in terms of their influence on theresidual current, for example, in accordance with the hypothesisformulated above, do not accumulate at the particle surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a and FIG. 1 b are pictorial representations of a scanningelectron micrograph (SEM) image of a preferred powder according thepresent invention, which includes agglomerated spherical primaryparticles; and

FIG. 2 is a graphical representation of a pore size distribution curvefor the powder of FIG. 1, in which the Log of Differential Intrusion isplotted against Pore Size Diameter.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to niobium suboxide powders containing 100 to 600ppm of magnesium and/or 50 to 400 ppm of molybdenum and/or tungsten.Niobium suboxide powders which contain both 100 to 600 ppm of magnesiumand 50 to 400 ppm of molybdenum and/or tungsten are preferred.

The magnesium content is particularly preferably between 150 and 400ppm, and the molybdenum and/or tungsten content is particularlypreferably between 60 and 250 ppm. Molybdenum is preferred overtungsten, in particular in combination with magnesium.

Suitable niobium suboxides are those of formula NbO_(x), where x isbetween 0.7 and 1.3, corresponding to an oxygen content of between 10.8and 18.3% by weight; it is preferable for x to be between 1 and 1.033,corresponding to an oxygen content of from 14.7 to 15.1% by weight. Anoxygen content which is slightly above the stoichiometric composition ofx=1 is particularly preferred.

The levels of further impurities, with the exception of standarddopants, such as nitrogen and phosphorus, should be as low as possible.Levels or alloying constituents of tantalum are harmless, provided thatthey replace niobium in accordance with the formula (Nb, Ta)O_(x).Particularly preferred niobium suboxide powders have levels of Fe, Cr,Ni, Cu, alkali metals and fluoride and chloride of in each case lessthan 15 ppm. Furthermore, it is preferable for the sum of these levelsto be less than 35 ppm. The carbon content of the niobium suboxidepowders according to the invention should preferably be less than 40ppm.

A nitrogen content of from 10 to 500 ppm is advantageous.

Phosphorus contents in the niobium suboxide powders according to theinvention are not generally harmful. In niobium and tantalum metalpowders, phosphorus is used to lower the sintering activity duringproduction of the secondary structures and of the anode structure. Inmost cases, however, it is generally undesirable to lower the sinteringactivity with powders according to the invention. Phosphorus contents ofless than 10 ppm are preferred according to the invention. The powdersmay if appropriate be treated with phosphoric acid, ammonium hydrogenphosphate or ammonium phosphate prior to the sintering of the anodestructure.

Further, albeit less critical impurities, comprising Al, B, Ca, Mn andTi, preferably form less than 10 ppm, and there is also preferably lessthan 20 ppm of Si.

The niobium suboxide powders according to the invention preferablycomprise agglomerated primary particles with a mean cross-sectionaldimension of from 0.3 to 1.5 μm, particularly preferably between 0.4 and1 μm. The primary particles may be in the form of beads, platelets,filaments or in other forms. It is important for the smallestcross-sectional dimension (if the shape is other than spherical) to bein the range between 0.3 and 1.5 μm.

The primary particle agglomerates preferably have a particle sizedistribution determined in accordance with ASTM B 822 (“Mastersizer”;wetting agent Daxad 11) which is defined by a D10 value of 50 to 100 μm,a D50 value of from 150 to 200 μm and a D90 value of from 250 to 350 μm.The sponge-like agglomerates have a round to oval cross section and goodflow properties (in accordance with Hall, ASTM B 213) of less than 60sec/25 g. The bulk density (in accordance with Scott, ASTM B 329) isbetween 0.9 and 1.2 g/cm³ (14.8 to 19.7 g/inch³). The specific surfacearea (BET, ASTM D 3663) is between 0.85 and 2.5 m²/g, particularlypreferably between 1 and 1.5 m²/g.

FIG. 1 shows an SEM image of a preferred powder of this type whichcomprises agglomerated spherical primary particles.

The excellent pressing and sintering properties of the preferred niobiumsuboxide powders also results from the stability of the sinteredbridges, which can be established by means of the temperature during aagglomeration. One measure of this is the change in the particle sizedistribution after the agglomerates have been treated in an ultrasoundbath. After the preferred agglomerates have been treated for 15 minutesin an ultrasound bath, a bimodal particle size distribution withpronounced frequency maxima between 2 and 10 μm (secondary maximum), onthe one hand, and between 90 and 200 μm (primary maximum), on the otherhand, is established. The D10 value (Mastersizer, ASTM B 822) is between1.5 and 3.0 μm, the D50 value is between 20 and 60 μm and the D90 valueis between 70 and 130 μm.

The powders which are preferred in accordance with the invention have aporosity, determined by mercury intrusion, of from 50 to 65% by volume,particularly preferably 53 to 60% by volume. More than 90% of the porevolume is formed by pores with a diameter of between 0.2 and 3 μm. Thebroad pore volume distribution curve plotted against the pore diameterhas steep flanks with a minimum in the region of twice the primaryparticle diameter.

FIG. 2 shows a pore size distribution curve of this type for the powdershown in FIG. 1, with a deficit of pores in the range from 1.1 to 1.2μm.

The niobium suboxide powders which are preferred in accordance with theinvention allow capacitors with a residual current of less than 0.2nA/μFV to be produced using the standard method. Residual currents of aslow as 0.03 nA/μFV are achieved.

Accordingly, the invention also relates to capacitors with a niobiumsuboxide anode and a niobium pentoxide barrier layer which have aresidual current of less than 0.2 nA/μFV, the niobium suboxide beingdoped with magnesium, molybdenum and/or tungsten. Preferred features ofthe capacitors according to the invention correspond to the preferredfeatures of the niobium suboxide powders described above.

Capacitors according to the invention have specific capacitances of50,000 to 200,000 μFV/g, preferably 70,000 to 150,000 μFV/g.

The capacitors can be produced as follows:

The powder is pressed to a pressed density of 2.3 to 3.5 g/cm³ around aniobium or tantalum wire inserted into the press mould, to form powderpreforms. Pressed bodies with a very favourable pressed strength areobtained. By way of example, the standardized measurement carried out oncylindrical pressed bodies (without wire) with a diameter of 5.2 mm anda height of 5.1 mm using a weighed-in quantity of 301 mg of niobiumsuboxide powder, after pressing to a density of 2.8 g/cm³, is stableunder an applied weight of from 0.5 to 1 kg.

The pressed bodies containing the contact wire are then preferablysintered in a niobium or tantalum boat at 1100 to 1500° C. for asintering holding time of from 15 to 25 minutes, preferably approx. 20minutes, under a high vacuum at 10⁻⁸ bar. The sintering temperature isselected in such a way that the capacitor surface area, which cansubsequently be calculated from the capacitance, is still 65 to 45% ofthe specific surface area measured for the powder. The optimum sinteringtemperature and sintering holding time can be determined by sinteringthe pressed body described above for the determination of the pressedstrength. The sintering temperature and time are preferably selected insuch a way that this pressed body is able to withstand an applied loadof from 8 to 18 kg.

In the context of the present disclosure, the residual current andcapacitance were determined in the following way:

The sintered anode structures were formed in an aqueous electrolytecomprising 0.1% by weight strength H₃PO₄ at 85° C. and a forming currentof 150 mA/g up to a forming voltage of 30 V and over a final formingtime (virtually current-free) over 120 minutes.

The capacitance and residual current were measured by immersing thecapacitors in an aqueous electrolyte comprising 18% strength by weightH₂SO₄ at a temperature of 25° C. and an AC voltage of 70% of the formingvoltage (21 V) and 120 Hz, with a superimposed bias voltage of 10 V,after a charging time of 3 minutes.

The powders according to the invention can be produced using standardprocesses. The standard metallurgical reaction and alloying process,according to which, as in the present case, a mean oxide content is setby exposing a highly oxidized precursor and a nonoxidized precursor, ina nonoxidizing, preferably reducing atmosphere, to a temperature atwhich an oxygen concentration balancing takes place, is preferred.Processes other than this solid-state diffusion process are conceivable,but they require control and monitoring functions which are in technicalterms almost intractable at acceptable outlay. Therefore, according tothe invention it is preferable to use a high-purity, commerciallyavailable niobium pentoxide and for the latter to be mixedstoichiometrically with high-purity niobium metal, both in powder form,followed by treatment at a temperature of from 800 to 1600° C. in an H₂atmosphere for several hours. It is preferable for both the pentoxideand the metal to have primary particle sizes which, after the oxygenbalancing, correspond to the desired primary particle size of below orslightly above 1 μm (minimum) cross-sectional dimension.

According to the invention, it is preferable for the doping withmagnesium, molybdenum and/or tungsten to be carried out at the latestbefore or during, particularly preferably before, the oxygen exchangebetween the oxide component and the metal component.

To avoid contamination, it is preferable for all the reactors andvessels, such as crucibles, boats, grates, meshes, etc. which come intocontact with niobium or niobium oxides at elevated temperature to bemade from or lined with preferably niobium or tantalum.

The niobium metal required for the oxygen exchange with niobiumpentoxide is preferably produced by reduction of high-purity niobiumpentoxide to form the metal. This can be effected aluminothermically byigniting an Nb₂O₅/Al mixture and washing out the aluminium oxide whichis formed and then purifying the niobium metal ingot by means ofelectron beams. The niobium metal ingot obtained after reduction andelectron beam melting can be embrittled using hydrogen in a known wayand milled, producing plateletlike powders. In this case, the doping isadvantageously performed by adding the doping metals to the melt.

The preferred process for producing the niobium metal follows thedisclosure of WO 00/67936 A1. According to this preferred two-stageprocess, the high-purity niobium pentoxide powder is firstly reduced bymeans of hydrogen at 1000 to 1600° C., preferably up to 1400° C., toform the niobium dioxide of approximately formula NbO₂, and is thenreduced to the metal using magnesium vapour at 900 to 1100° C. Magnesiumoxide which is formed in the process is washed out by means of acids.For magnesium doping which is sufficient in accordance with theinvention, it is generally sufficient to leave out the final acid washfrom the teaching of WO 00/67936 A1. However, it is preferable to addpreferably MgO to the metal component and/or oxide component prior tothe oxygen exchange reaction. For the molybdenum and/or tungsten doping,it is advantageously possible to carry out an impregnation in molybdicand/or tungstic acid solution prior to the reduction of the pentoxide toform the metal. The person skilled in the art will be readily familiarwith further doping options. By way of example, MoO₃ and/or WO₃ powdersmay be added to the niobium pentoxide powder or niobium dioxide powder.The doping both with Mg and with Mo/W, or the preferred mixed dopingwith both Mg and Mo and/or W, with Mo being preferred over W,particularly preferably takes place as early as during production of theniobium pentoxide, for example through addition of the correspondingdopants, preferably the oxides, to the Nb(OH)₅, which is converted intoniobium pentoxide by heating in a manner which is known per se.

EXAMPLES 1 TO 9

The starting material is a niobium pentoxide powder produced bycalcining a niobium hydroxide which has been obtained by precipitationfrom an H₂NbF₇ solution by means of aqueous ammonia solution. Thechemical analysis was as follows:

Al <1 ppm As   <1 ppm Ca <1 ppm Cl   <3 ppm Co <0.1 ppm   Cr <0.3 ppm Cu0.4 ppm  F   51 ppm Fe <1 ppm K <0.5 ppm Mg <1 ppm Mo <0.3 ppm Na  2 ppmNi <0.2 ppm Si  8 ppm Ta  <10 ppm Ti <1 ppm V   <1 ppm W <0.5 ppm   Zr <0.5 ppm.

Where the “<” sign is used for the analysis values, the concentrationindication in each case characterizes the detection limit of theanalysis method, or the content can be characterized as below thedetection limit on the basis of the analysis accuracy.

The powder agglomerates comprised very uniform sintered, sphericalprimary particles with a mean diameter of 0.6 μm.

The BET specific surface area was 2.4 m²/g. 97.5% by weight of theagglomerates were smaller than 300 μm (sieve analysis).

In each case one quantity of the powder was doped with the quantity(ppm) of Mg, Mo and/or W given in Table 1 below by the addition of MgOpowder, MoO₃ powder and/or WO₃ powder followed by further calcining inair.

TABLE 1 Mg Mo W Example ppm ppm ppm 1 (Comp.) — — — 2 250 — — 3 — 200 —4 — — 220 5 200 150 — 6 180 170 — 7 170  60  60 8 100  30 — 9 200  50 —

A part of each of the powders 1 to 9 was firstly reduced by calcining at1380° C. under hydrogen to form the NbO₂. The NbO₂ was then placed ontoa mesh of niobium wire, beneath which, in a vessel made from niobiummetal, was approximately 1.4 times the stoichiometric quantity ofmagnesium chips, based on the oxygen content of the NbO₂. This wasfollowed by heating to 970° C. under an argon atmosphere at a pressureof approximately 1050 mbar. After 6 hours, the temperature was in eachcase slowly cooled with gradual introduction of air for passivationpurposes.

After sieving through a sieve with a mesh width of 300 μm, the powderwas repeatedly leached with 8% strength by weight sulphuric acid, washedand dried in order to remove the MgO formed.

The surface of the metal powder was greatly roughened. Depending on thebatch, the specific surface area was from 4.5 to 5 m²/g, with only aslightly smaller primary structure dimension of from 0.45 to 0.55 μm.

Each of the metal powders was mixed in a molar ratio of 3:1 with thestarting niobium pentoxide in a quantity which was such that the meancomposition of the mixture formally corresponded to the formula NbO. Themixtures were each heated slowly to 1400° C. for four hours in ahydrogen atmosphere at 1050 mbar, cooled slowly and passivated.

The NbO obtained had the contents of doping elements shown in Table 2.

The primary particle diameter determined under a scanning electronmicroscope was 0.5 to 0.65 μm. The D10 value was 50 to 70 μm, the D50value was 170 to 190 μm and the D90 value was 270 to 295 μm. Thespecific surface area was between 1 and 1.15 m²/g.

TABLE 2 Mg Mo W Example ppm ppm ppm 1 (Comp.) 80 — — 2 350 — — 3 84 262— 4 82 — 289 5 303 187 — 6 310 205 — 7 294  74  69 8 178  53 — 9 366  81—

The further impurities were substantially unchanged. The levels ofharmful impurities were as follows:

C 24 ppm  Cl <1 ppm  Cr 2 ppm Cu 0.4 ppm   F 2 ppm Fe 6 ppm K <1 ppm  Na2 ppm Ni 2 ppm

Anodes with a diameter of 3.6 mm and a length of likewise 3.6 mm werepressed from the powders around a tantalum wire placed into the pressmould, with a thickness of 0.3 mm, at a pressed density of 2.8 g/cm³,followed by sintering under a high vacuum at 1460° C. for 20 minutes.

The anodes were formed in an electrolyte comprising 0.1% strength byweight phosphoric acid at a temperature of 85° C. and a forming currentof 150 mA/g up to a forming voltage of 30 V, which was held for twohours after the current had decayed.

The capacitance and residual current of the anode bodies provided with abarrier layer of niobium pentoxide by the forming were measured by thecounterelectrode being simulated by an 18% strength by weight sulphuricacid at 25° C. The measurements were carried out at a voltage of 21 V(70% of the forming voltage), a frequency of 120 Hz and a bias voltageof 10 V after a charging time of 3 minutes. The measurement results arecompiled in Table 3.

TABLE 3 Spec. Spec. residual capacitance current Example μFV/g nA/μFV 1(Comp.) 70846 2.3 2 72483 0.08 3 71925 0.12 4 68569 0.14 5 71896 0.03 672371 0.02 7 70478 0.05 8 77746 0.11 9 79112 0.04

1. A niobium suboxide powder comprising 100 to 600 ppm of magnesium. 2.A niobium suboxide powder comprising 50 to 400 ppm of a member selectedfrom the group consisting of molybdenum, tungsten and combinationsthereof.
 3. A niobium suboxide powder comprising 100 to 600 ppm ofmagnesium, and 50 to 400 ppm of a member selected from the groupconsisting of molybdenum, tungsten and combinations thereof.
 4. Theniobium suboxide powder of claim 1 having a magnesium content of 150 to400 ppm.
 5. The niobium suboxide powder of claim 2 having a molybdenumcontent of 60 to 250 ppm.
 6. The niobium suboxide powder of claim 1comprising a member selected from the group consisting of Fe, Cr, Ni,Cu, alkali metals, fluoride, chloride and combinations thereof, whereineach member is independently present in an amount of less than 15 ppm.7. The niobium suboxide powder according to claim 6 wherein said memberselected from the group consisting of Fe, Cr, Ni, Cu, alkali metals,fluoride, chloride and combinations thereof, is present in an amounttotalling less than 35 ppm.
 8. The niobium suboxide powder of claim 3having a carbon content of less than 40 ppm.
 9. The niobium suboxidepowder of claim 3 having a nitrogen content of 10 to 500 ppm.
 10. Theniobium suboxide powder of claim 1 wherein the mean composition of saidniobium suboxide is represented by the formula, NbO.sub.x wherein0.7<x<1.3.
 11. The niobium suboxide powder of claim 1, comprisingagglomerated primary particles having a diameter of from 0.3 to 1.5 μm.12. A niobium suboxide anode comprising a sintered powder of the niobiumsuboxide powder of claim
 1. 13. A solid electrolyte capacitor comprisingthe niobium suboxide anode of claim 12, and a barrier layer comprisingniobium pentoxide.
 14. The niobium suboxide powder of claim 1 having acarbon content of less than 40 ppm.
 15. The niobium suboxide powder ofclaim 1 having a nitrogen content of 10 to 500 ppm.
 16. The niobiumsuboxide powder of claim 3 wherein the mean composition of said niobiumsuboxide is represented by the formula, NbO_(x) wherein 0.7<x<1.3.
 17. Aniobium suboxide anode comprising a sintered powder of the niobiumsuboxide powder of claim
 3. 18. A solid electrolyte capacitor comprisingthe niobium suboxide anode of claim 17, and a barrier layer comprisingniobium pentoxide.
 19. The niobium suboxide powder of claim 2, having amagnesium content of 150 to 400 ppm.
 20. The niobium suboxide powder ofclaim 3, having a magnesium content of 150 to 400 ppm.